A rotor for an electricity generator

ABSTRACT

A rotor ( 10 ) for a hydro-powered electricity generator. The rotor ( 10 ) includes a hub ( 12 ) and a plurality of blades ( 16 ). The hub ( 12 ) has a circular cross sectional shape and a longitudinal rotational axis ( 14 ). The plurality of blades ( 16 ) each have a proximal root ( 16   a ) and a distal tip ( 16   b ). Each of the blade roots ( 16   a ) are mounted to the hub ( 12 ) at the widest part thereof (D 1 ). The ratio between the diameter of the tips ( 16   b ) of the blades to the diameter of the widest part (D 1 ) of the hub ( 12 ) is less than about 2:1.

FIELD OF THE INVENTION

The present invention relates to a rotor for an electricity generator.

The invention has been primarily developed for use in a rotor for ahydro-powered electricity generator. Such generators are used to convertkinetic energy from flowing fluids, such as water and wind, toelectrical power.

BACKGROUND OF THE INVENTION

Kinetic energy in flowing fluids, such as water and wind, is a knownalternative to energy sources such as bio-fuels and fossil fuels forgenerating power. Unlike, for example, bio- and fossil fuels which, whenused in electrical power generation, go hand-in-hand with emission ofharmful combustion gasses into the atmosphere, generation of power byusing flowing fluids has no or very little adverse effects on theatmosphere.

Known installations for harvesting wind power generally have low runningcosts, however they tend to be expensive to install and have relativelylow generation capacity. Known installations for harvesting hydropower,for example tidal power, on the other hand, have relatively highergeneration capacity.

Known hydro-powered electricity generators typically have a rotorcomprising a central hub to which is attached two or more outwardlyextending blades. The rotor is connected by a drive shaft to a rotarywork to electrical power converter (i.e. a generator). Fluid flowingpast the rotor blades causes it to rotate which in turn causes therotation in the converter and the generation of electrical power.

Known rotors have a relatively small diameter hub and relatively longand slender blades. The blades also have a relatively high aspect ratio(being the ratio of the blade length to the blade width). Such bladesare prone to high operating loads and subject to extreme bending momentsin turbulent fluid flow. This typically results in broken blades.

OBJECT OF THE INVENTION

It is an object of the present invention to substantially overcome, orat least ameliorate, the above disadvantage.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a rotor for ahydro-powered electricity generator, the rotor including:

-   -   a hub with a circular cross sectional shape and a longitudinal        rotational axis,    -   a plurality of blades, each having a proximal root and a distal        tip, each of the blade roots being mounted to the hub at the        widest part thereof,    -   wherein the ratio between the diameter of the tips of the blades        to the diameter of the widest part of the hub is less than about        2:1.

Preferably, the ratio between the diameter of the tips of the blades tothe diameter of the widest part of the hub is between about 1.2:1 and2:1.

Preferably, the ratio between the diameter of the tips of the blades tothe diameter of the widest part of the hub is about 1.5:1 or 1.6:1.

In one embodiment, the diameter of the tips of the blades is between 3.6and 4.8 metres and the diameter of the widest part of the hub is 2.4metres.

In another embodiment, the diameter of the tips of the blades is between30 and 32 metres and the diameter of the widest part of the hub is 20metres.

The profile radius of the hub surface, in the region where each of theblade roots are mounted to the hub, is preferably between ⅙th of andequal to the radius of the widest part of the hub.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexamples only, with reference to the accompanying drawings in which:

FIG. 1 is a front view of a first embodiment of a rotor;

FIG. 2 is a perspective view of the rotor shown in FIG. 1 with streamlines; and

FIG. 3 is cross sectional side view of a hydro-powered electricitygenerator with a second embodiment of a rotor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a rotor 10 for a hydro-powered electricity generatorsuitable for installation in a tidal flow environment. The rotor 10includes a hub 12 with a circular cross sectional shape and alongitudinal rotational axis 14. The rotor 10 also includes 7equiangularly spaced apart blades 16. The hub 10 is formed from glassreinfornced plastic (GRP) or metal skins and the blades 16 are formedfrom carbon fibre metal composites.

Each of the blades 16 has a proximal root 16 a and distal tip 16 b. Eachof the blades 16 are mounted to the hub 14, at their roots 16 a, at thewidest part of the hub 14. The diameter of the widest part of the hub 14is shown as diameter D1. The diameter of the tips 16 b of the blades 16is shown as diameter D2. In the embodiment shown, the ratio betweendiameters D2:D1 is about 1.4:1.

FIG. 2 shows the rotor 10 relative to fluid flow stream lines 18 whichdemonstrate that as the fluid flows around the hub 12 its velocityincreases. As the fluid accelerates and the local velocity increases,the local pressure decreases. This pressure reduction causes the fluidto remain concentrated around the hub 12. As a result, the energy in afree stream of the fluid is concentrated in the region of the blades 16.

Another way of describing the above D2:D1 ratio is that the diameter ofthe hub 12 is relatively large compared to the length of the blades 16.The relatively large hub diameter D1 advantageously serves the dualfunction of: 1. concentrating the energy in the passing water stream;and 2 supporting a relatively greater number of smaller and strongerblades 16, which each have a lower aspect ratio.

In relation to the latter issue, the bending moment at the root is afunction of the aspect ratio of the blade. For example, a blade with anaspect ratio of 8:1 will have a stress value in the root that is 16times higher than the same blade with an aspect ratio of 4:1. In a known3-blade rotor with a relatively small diameter hub, the blades can onlyhave a limited chord length at the root due to the diameter restrictionof the hub. This restriction of chord length means that the blade rootthickness must be increased, to provide sufficient strength, over thatotherwise required for an ideal foil section.

A relatively longer blade mounted to a relatively smaller hub alsoresults in a lower apparent velocity for a given RPM and a lower torqueradius.

A thicker root, especially in the lower ⅓^(rd) of the blade, combinedwith the lower apparent velocity and the lower torque radius, results ina lowered contribution to the total power of such a (known) 3-bladerotor. This is due to the fact that the outer ⅓^(rd) of the blade in thesmaller hub/larger 3-blade configuration does 63% of the work. This is acombination of the swept area of the outer 30% of the blade, whichconstitutes 56% of the total surface area, and the inner 30% of theblade producing negligible power.

In contrast, the configuration of the rotor 10 (i.e. relatively largerhub 14, relatively shorter blades 16, relatively large number of blades16) redirects and concentrates the fluid flow in the inner ⅔ region andaccelerates it through the outer ⅓^(rd) region where 100% of the powercan be extracted. This advantageously means that the blades 16 areoperating at maximum capacity, while also experiencing a lower stressloading.

Put another way, the D2:D1 ratio of the rotor 10 places the blades 16 ina zone of acceleration around the hub 12 with an ideal blade length forthe blades 16 to operate in that zone. If the blades are too longrelative to the hub diameter then the blades tips instead operate in aregion with no fluid acceleration and therefore do not contributepositive torque.

FIG. 3 shows a hydro-powered electricity generator 30 with a secondembodiment of rotor 32. The rotor 32 has a hub 34 and ten blades 36.FIG. 3 also shows blade root mounting beams 38, a blade mounting hub 40,a fixed main spindle 42, a drive shaft 44, a gear box 46, a support beam48, a water seal 50, bearings 52 and a rotary electrical generator 54.The beam 48 is used to connect the generator 30 to a floating deploymentrig (not shown).

Also shown on FIG. 3 is radius R, being the profile radius hub 34 in theregion where the hub 34 and the blades 36 are connected. In thepreferred configuration shown, the radius R is ⅙ the radius of the hub34. This particular ratio maximises flow acceleration while avoidingturbulence.

One preferred form of the generator 30 has the following specifications:

Hub diameter D1: 2.4 meters

Blade tip diameter D2: 4.8 to 3.6 meters

Power generation range: 50 to 300 kWs

Flow velocity range: 1.2 to 4.2 m/sec

Blade tip diameter to hub diameter ratio: 2:1 to 1.5:1

Another preferred form of the generator 30 has the followingspecifications:

Hub diameter D1: 20 meters

Blade tip diameter D2: 32 to 30 meters

Power generation range: 0.5 to 5 MWs

Flow velocity range: 1.2 to 4.0 m/sec

Blade tip diameter to hub diameter ratio: 1.6:1 to 1.5:1

There are several advantages for hydro-powered generators due to the(relatively larger) diameter hub to (relatively smaller) diameter bladeratios described above.

Firstly, the energy in the fluid stream is concentrated and acceleratedacross a set of small blades, which improves the efficiency of therotor.

Secondly, the total volume of the multiple (e.g. 7) smaller blades isless than the volume of a small number of (e.g. 3) large blades, whichlowers manufacturing cost.

Thirdly, the smaller blades have a lower aspect ratio, which equates toa lower bending moment in the blade root, and a lower probability ofblade breakage.

Fourthly, the incident velocity and the incident angle of the flow ontothe smaller blades is closer to a uniform value across the span of theblades. This equates to near zero twist in the blade across its span,and allows the blades to be articulated in pitch control without anyperformance losses induced by blade twist. Further, the ability toadjust the pitch during operation means the rotor can be run at aconstant rpm independent of the flow stream velocity. This allows thegenerator to be run at a constant rpm connected directly to theelectrical grid thereby negating the cost of an electrical frequencyinverter drive system.

Fifthly, rotors operating in fast flowing tidal flows are subject tohigh levels of turbulence in the stream. The action of the flowacceleration of the water around the larger hub reduces the level ofturbulence into the blade region. This improves the survivability of theblades in highly turbulence environments.

Although the invention has been described with reference to preferredembodiments, it will be appreciated by person skilled in the art thatthe invention may be embodied in other forms.

1. A rotor for a hydro-powered electricity generator, the rotorincluding: a hub with a circular cross sectional shape and alongitudinal rotational axis, a plurality of blades, each having aproximal root and a distal tip, each of the blade roots being mounted tothe hub at the widest part thereof, wherein the ratio between thediameter of the tips of the blades to the diameter of the widest part ofthe hub is less than about 2:1.
 2. The rotor as claimed in claim 1,wherein the ratio between the diameter of the tips of the blades to thediameter of the widest part of the hub is between about 1.2:1 and 2:1.3. The rotor as claimed in claim 1, wherein the ratio between thediameter of the tips of the blades to the diameter of the widest part ofthe hub is about 1.5:1 or about 1.6:1.
 4. The rotor as claimed in claim1, wherein the diameter of the tips of the blades is between 3.6 and 4.8metres and the diameter of the widest part of the hub is 2.4 metres. 5.The rotor as claimed in claim 1, wherein the diameter of the tips of theblades is between 30 and 32 metres and the diameter of the widest partof the hub is 20 metres.
 6. The rotor as claimed in claim 1, wherein theprofile radius of the hub surface, in the region where each of the bladeroots are mounted to the hub, is between ⅙th of and equal to the radiusof the widest part of the hub.